10 



HYDRAULICS. 



by atmospheric pressure, comprises all 

 those machines to which the name of 

 pump is more particularly applied. Of 

 pumps there are several varieties ; but 

 the simplest and most common is the 

 ordinary lift, or Household Pwrap,which 

 depends chiefly on the pressure of the 

 atmosphere for its action. This useful 

 machine is one of great antiquity, its 

 invention being ascribed by Vitmvius to 

 Ctesebes of Alexandria, before-men- 

 tioned ; but the principles upon which 

 it acts were not understood until long 

 afterwards, as appears by the very lame 

 explanation of them that is attempted 

 by Galileo towards the beginning of the 

 seventeenth century. The nature of at- 

 mospheric pressure was not, however, 

 at this time at all understood ; and it is 

 a curious fact, that the experiments 

 made upon this now common machine 

 should have led to the invention of the 

 barometer, by which the variations of 

 the atmosphere have since been so ac- 

 curately investigated. 



The form and construction of the com- 

 mon lift-pump is shown in section at 

 jfig. 7, in which m m is the cylinder or 



fiff-7- 



barrel, n an air-tight piston, which moves 

 or works within it, by means of the pis- 



ton-rod o, moved by the lever pp, or 

 any other contrivance ; q is the suction 

 or feeding pipe, descending into the jar 

 of water r r, which would be a well or 

 other reservoir in an actual pump, s a 

 valve at the bottom of the barrel cover- 

 ing the top of the feeding-pipe, and t 

 a valve in the piston, both which valves 

 open upwards ; u u is an open-topped 

 receiver for supporting the pump above 

 the jar of water r r. Raising the piston 

 n from the bottom to the top of the bar- 

 rel, will produce a vacuum in the bar- 

 rel between n and *, and the pressure of 

 the air upon the surface of the water at 

 r r, will force a quantity of that water 

 up q, through the valve s, into the in- 

 terior of the barrel, where it will be re- 

 tained, because it cannot pass back 

 again through s ; when the piston n is 

 lowered, it can pass through the water 

 previously raised, because its valve t 

 will open, and thus it gets to the bottom of 

 the barrel. On raising the piston a second 

 time, the water, which has so passed 

 through it, will be carried up by it. into 

 the cistern o, from whence it will be dis- 

 charged by the spout v, while a new 

 vacuum is forming between n and s, 

 which will, of course, be supplied as 

 before with water ; and thus it will ap- 

 pear, that the common water-pump is 

 rather a pneumatic, than an hydraulic 

 machine, because it raises water only 

 by the production of a vacuum within 

 the working barrel ; in consequence of 

 which, the external atmospheric pres- 

 sure is called into action, and forces the 

 water of the well up the suction-pipe. 

 The consequence of this is, that if the 

 piston, at its greatest elevation, should 

 at any time exceed the distance of thirty- 

 three feet from the surface of the water 

 in the well, the working of the pump 

 may not produce a sufficiently perfect 

 vacuum to raise the water. 



It may not be amiss to notice a frequent 

 eiTor in the construction of pumps, which 

 is very detrimental to their action, 

 namely, making the feeding-pipe, or that 

 pipe which proceeds from the water to 

 be raised, to the bottom of the working 

 barrel, of too small a capacity, under a 

 notion, that if this pipe is large, the pis- 

 ton in ascending will have to raise and 

 draw after it a much thicker column of 

 water, and consequently a much greater 

 load than is necessary. The fallacy of 

 this supposition is clearly shown in the 

 Treatise upon Hydrostatics ; for whe- 

 ther a column of water be pressing 

 downwards upon a piston, or be under- 



